Wegener's granulomatosis (WG) and microscopic polyangiitis (MPA) are systemic autoimmune small vessel vasculitides. Their clinical manifestations are very heterogeneous. In the case of WG they may range from indolent cavitating pulmonary nodules to massive life threatening alveolar hemorrhage with or without associated glomerulonephritis. Essentially any organ may be affected, and it remains unclear what determines the predominant organ involvement observed in individual patients. These diseases are fatal if untreated. Immunosuppressive agents have significantly improved the prognosis, but are associated with substantial side effects. WG and MPA are associated with circulating autoantibodies, antineutrophil cytoplasmic antibodies (ANCA). The most prominent target antigen for ANCA in WG is the neutrophil azurophil granule serine protease, proteinase 3 (PR3). Clinical observations suggest that the type of ANCA may influence the prognosis and spectrum of clinical presentations. ANCA have been implicated in the pathogenesis of these diseases. Despite very compelling evidence derived from clinical observations, in vitro studies, and attempts to develop valid animal models, the exact mechanisms by which ANCA influence the heterogeneous disease manifestations remain elusive. A detailed understanding of the specific pathogenetic interactions of PR3-ANCA with its target antigen will provide the foundation for potential novel therapeutic approaches with less side effects. Therefore, we hypothesize that the interactions of PR3-ANCA with its target antigen are pathogenic. We further hypothesize that different subtypes of PR3-ANCA reacting with different epitopes on PR3 modulate specific PR3 functions and thereby affect clinical disease expression. To address this hypothesis, we will categorize PR3-ANCA subtypes, identify their target epitopes on the PR3 molecule, and determine their impact on PR3 function and specific organ manifestations of the disease (specific aim 1). To prove the pathogenicity of PR3-ANCA in vivo, we will develop mouse models based on the interactions of murine ANCA with their species-specific target antigen (specific aim 2). Our studies are fundamental to the understanding of the pathogenetic role of PR3-ANCA in pulmonary vasculitis, and for the development of novel mechanism-based therapeutic approaches for these devastating disease states.